Hi Stephanie - when I teach changing units, I include dimensional analysis
as one of the ways to do it. I think the key is to relate the process back
to concepts the students are familiar with - in this case proportions and
multiplying by 1, because that's what you're really doing when you multiply
by the conversion factor. For example, suppose I need to change 254 cm to m.
My conversion factor is
1 m
________ . Kids always wonder why I picked this form.
100 cm
I model it by thinking out loud: "I need 2 equivalent measurements (so the
fraction equals 1) that contains cm and m. I know 1 m = 100 cm and I need
the cm in the denominator so the cm units will be the fraction 1 (divide
out). I need to check that I'm using a conversion that really equals 1 and
that I'll get m left when I divide everything."
1 m 254 cm x 1 m 254 cm m
254 cm x ------ = -------------- = --- x --- x ---
100 cm 100 cm 100 cm 1
= 2.54 m
If you haven't tried modeling the process out loud and relating it back to
multiplying by 1, you might try this. It usually works for my students.
-Jenny, for the Teacher2Teacher service